Hanoi weather is very changeable. Although the city experiences four distinct seasons it would be fair to summarize the year in terms of summer (from May to September) when it is very hot and wet and winter (November to March) when it is cold and dry.
What about October and April I hear you ask? Well in this humid, tropical climate in which monsoons are common pretty much any conditions can occur in these most unpredictable of months.
We visited in November/December when the skies were clear most of the time and the air quite fresh. Very pleasant for tourism purposes.
Below is the Hanoi Weather for today and tomorrow. Click anywhere on it for a longer range forecast. Under it you will find two graphs showing the average monthly temperature and rainfall throughout the year.